This invention relates to an improved system for controlling the flow of vaporized liquid transported by a carrier gas from a reservoir of the liquid to a using system. The control system improves the accuracy with which a uniform mass flow of the vapor is delivered to the using system and is particularly useful in the vapor deposition of high purity, liquid source materials such as in the manufacture of semiconductor devices and fiber optic materials.
The fabrication of semiconductor electronic devices, for example, includes many steps which require the transportation of particular atoms or molecules to the surfaces of wafer substrates which are usually maintained at elevated temperatures. In many of the steps, the most common method for accomplishing this is to transport the vapors from a reservoir of liquid chemical by passing a carrier gas stream through the liquid to vaporize the liquid and carry its vapor to a reaction chamber of the using system. Acceptable performance depends upon maintenance of accurate vapor delivery rates and extremely low levels of impurities, particularly metallics.
Typically, a container is partially filled with an ultra-high purity, liquid source material, and a suitable carrier gas stream is bubbled through the liquid to vaporize a portion of the liquid and transport the vapor from the container to the point of use. The previous methods of vapor flow control that have been customarily used for such bubblers are the thermal-conductivity mass flow meter and the temperature controlled vaporizer bubbler; however, neither method has been entirely satisfactory.
The thermal conductivity mass flow meter monitors the vapor flow from a liquid source bubbler by taking the ratio of the thermal-conductivity of the carrier gas and vapor mixture flowing out of the bubbler, to the thermal-conductivity of the carrier gas flowing into the bubbler (see for example U.S. Pat. No. 3,650,151). The main drawback of this method is the introduction of metallic contamination into the vapor stream. The design and construction of the thermal mass flow meter has necessitated the use of metallic parts, usually stainless steel, in the chemical vapor path. Because of the highly corrosive nature of many of the commonly used chemical vapors (especially in the presence of trace levels of moisture contamination) the metallic parts slowly deteriorate and the resulting metallic impurities are carried with the source vapor to the wafers. This leads not only to wafer contamination in the fabrication of semiconductors and low yield of acceptable wafers, but also to drift and failure in the mass flow controller caused by deterioration by the chemical. Thus, this type of apparatus loses its accuracy over a period of time. In addition the meters themselves are costly and must frequently be repaired and replaced. This is particularly so under conditions of high carrier gas flow, such as is encountered in fiber optic applications.
The temperature controlled bubbler method maintains constant vapor mass flow by closely controlling the bubbler temperature and the mass flow rate of the carrier gas stream. Recently introduced improved bubblers, such as those illustrated in U.S. Pat. Nos. 4,134,514 and 4,140,735 eliminate many contamination and deterioration problems by using only high-purity quartz and Teflon parts in contact with the vapors, and by eliminating chemical handling problems. The main drawback to this method has been fluctuations in the vapor mass flow such that the output has not always been sufficiently satisfactory and has required considerable trial and error adjustment of the carrier gas flow rate. As a result of inadequate controls, there is a significant and frequent loss of partially finished goods. Thus, although these devices do not suffer from deterioration caused by the chemicals used, they are not sufficiently accurate. In addition, there have been some reported instances of connections being broken or bubblers exploding because of improperly high pressures of the carrier gas streams. This results in danger to operating personnel because of the corrosive nature of the liquid source material, as well as the possible loss of the expensive work in progress, such as a batch of semiconductor wafers. Accordingly, a need exists for improved flow control of such a bubbler system. The present invention is concerned with such an improvement for a bubbler system of the temperature controlled type.